1. Field of the Invention
The present invention relates to lens sheets for rear projection screens for use in rear projection television sets.
2. Description of the Prior Art
As shown in FIG. 1, the rear projection screen for use in a rear projection television set is a combination of, at least, a Fresnel lens sheet 1 disposed on the light source 5 side and a lenticular lens sheet 2 disposed on the watcher side, and further a front panel 3 further disposed adjacent to the lenticular lens sheet on the watcher side. A mirror 4 is further disposed to reflect light rays from the light source 5 to the rear projection screen. These sheets are mounted in a rear projection television set nearly in a close contact with each other. The Fresnel lens sheet and the lenticular lens sheet include diffusion elements, by which images are focused. Therefore, if the distance between the sheets is broadened for some kind of cause, projected images become blurred. The major cause of broadening the distance between the sheets is the warp or deflection of sheets due to the change in their water absorption depending on the change of ambient humidity.
To prevent such a disadvantage, there have been proposed various techniques for reducing the warp or deflection of the sheets caused by the change of ambient conditions. For example, JP 11-072848A proposes to make the surface layers of the front panel and the Fresnel lens sheet, each layer facing to the lenticular lens sheet, from a material having a larger water absorption than those of the opposite layers. JP 2000-214533A proposes to control the thickness of sheet, the degree of warp or deflection and the saturated water absorptivity. JP 2002-207253A proposes to form polycarbonate resin or MS resin (MMA-styrene copolymer resin) on both surfaces of a PMMA (polymethyl methacrylate) resin layer.
The proposed prior art techniques intend to reduce the warp or deflection of each sheet or a stack of sheets due to ambient humidity, by forming surface layers of Fresnel lens sheet and lenticular lens sheet from a low water-absorptive resin or by forming the surface layers of a stack of Fresnel lens sheet/lenticular lens sheet from a low water-absorptive resin. However, a single-layered sheet having a layer structure different in both surfaces, such as a sheet described in JP 11-72848A, is likely to undergo warp or deflection due to water absorption during its transportation because of its asymmetry. When both surface layers are made of a low water-absorptive resin and have a large thickness as described in JP 2002-207253A, the amount of antistatic agent to be added to the surface layers is increased. If the surface layers are thin, the water absorptivity of sheet is governed by that of the intermediate layer, to increase the degree of warp or deflection.
To reduce the warp or deflection of Fresnel lens sheet and lenticular lens sheet due to water absorption (moisture absorption), it is effective to reduce the water absorptivity of whole sheet. Generally, the Fresnel lens sheet is constituted from a plastic substrate and a Fresnel lens made of an ultraviolet cured resin, which is laminated to the surface of the substrate. The Fresnel lens is composed of a series of annular lens sections taken out of a simple lens which are concentrically arranged on a surface, and exhibits the same effects as those of a simple lens although having a smaller thickness. The Fresnel lens sheet is produced by applying an ultraviolet curable resin on the surface of the substrate, pressing a metal or resin mold having a shape matching with the contour of the Fresnel lens surface against the applied resin, and irradiating ultraviolet light from the side opposite to the mold to cure the resin into the Fresnel lens shape. The thickness of the plastic substrate is usually from 1.0 to 4.0 mm, and the thickness of the lens portion made of the ultraviolet cured resin is about 100 μm. The adhesion strength between the ultraviolet cured resin and the plastic substrate is important, because they may be separated from each other during the assembling into television sets, etc. if the adhesion strength is low. The main cause to reduce the adhesion strength can be the polarities of the raw materials themselves. Urethane-based resins and acrylate-based resins are widely used as the ultraviolet curable resins. If the plastic sheet contains a low polarity component such as styrene, the adhesion strength between the plastic sheet and the ultraviolet cured resin is reduced.